The present invention relates to a substrate with one or more via and pad structures for use in circuit assemblies with high reliability and high yield in manufacturing.
FIG. 1 illustrates a printed wiring board (PWB) with a substrate 10 having several plated vias such as plated via 35. Each plated via connects to associated conductive layers of the substrate 10. The conductive layers are located on the top, bottom, and/or in the substrate 10. Another structure is the via and pad structure such as via and pad structure 20 shown without a component. Via and pad structure 20 includes a solder mask 24, and a conductive trace 26 connecting a conductive pad 22 and a plated via 25 with a via hole 28.
FIG. 1 also illustrates that when conductive pad 32 is close to the via hole 38 the solder mask 34 will not prevent the solder from being siphoned into the via hole 38. The small amount of solder remaining will form an unreliable solder joint 31 at the terminal end 37 of the component 33. This problem is referred to as solder wicking. Decreasing the inner diameter of the solder mask 34 may reduce the solder wicking, but will require that the solder mask 34 be precisely aligned on the plated via 12 to avoid covering the via hole 38 and trapping contaminants causing corrosion. Gold plating on the plated via 12 may decrease corrosion but will increase solder wicking by increasing the wettability of the plated via 12. Solder wicking can also misalign a component with respect to a conductive pad such as terminal end 29 of component 13 which is misaligned with respect to the conductive pad 21. Another approach to reduce solder wicking is to move the conductive pad 18 further from the via hole 8, but this requires a long conductive trace, such as that covered by solder mask 17, which increases trace inductance and degrades performance at high frequencies.
FIG. 2A is a perspective view and FIG. 2B a sectional view of a surface mount component 33 on a substrate 10. FIGS. 2A-2B illustrate the solder wicking problem that arises using conventional via and pad structure 30. The unreliable solder joint 31 is formed on the terminal end 37 of the component 33 because the solder mask 34 is not effective due to the capillary action of the via hole 38 in close proximity to the conductive pad 32.